High-fidelity imaging of the cosmic mass distribution

The stars and gas seen in galaxies account for only a few percent of
the gravitating material in the Universe. Most of the rest has
remained stubbornly invisible and is now thought to be made of a new
form of matter never yet seen on Earth. Researchers at the Max Planck
Institute for Astrophysics have discovered, however, that a
sufficiently big radio telescope could make a picture of everything
that gravitates which would rival the pictures made by optical
telescopes of everything that shines.

Fig. 1:
Radio telescopes should be able to make pictures of hydrogen gas clouds
just a few 100 million years after the Big Bang, before they turned into
galaxies. Optical telescopes can make pictures of the galaxies themselves
as far back as a few billion years after the Big Bang. Measurements of the
distortion of these pictures by the gravity of foreground matter allow
telescopes of both types to make images of everything that gravitates rather
than everything that shines.

Fig. 2:
Image of the mass distribution over a patch of sky about 1/4
the area of the Full Moon. These images were made by PhD student
Stefan Hilbert using the Millennium Simulation, the largest computer
simulation of cosmic structure formation ever carried out. The upper
panel represents the kind of image which could be made by a
low-frequency radio telescope with a diameter of 100 kilometers, using the
gravitational distortion of images of pregalactic structure in the
neutral hydrogen distribution. The lower panel represents the kind of
image which could be made for the same region of the sky using an optical telescope
in space to measure the gravitational distortion of distant galaxy
images. (In the second image, the contrast has been artificially increased by a
factor of three compared to the first image to make structures more
visible.)

As light travels to us from distant objects its path is bent slightly
by the gravitational effects of the things it passes. This effect was
first observed in 1919 for the light of distant stars passing close to
the surface of the Sun, proving Einstein's theory of gravity to be a
better description of reality than Newton's. The bending causes a
detectable distortion of the images of distant galaxies analogous to
the distortion of a distant scene viewed through a poor window-pane or
reflected in a rippled lake. The strength of the distortion can be
used to measure the strength of the gravity of the foreground objects
and hence their mass. If distortion measurements are available for
sufficiently many distant galaxies, these can be combined to make a
map of all the foreground mass.

This technique has already produced precise measurements of the
typical mass associated with foreground galaxies, as well as mass maps
for a number of individual galaxy clusters. It nevertheless suffers
from some fundamental limitations. Even a big telescope in space can
only see a limited number of background galaxies, a maximum of about
100,000 in each patch of sky the size of the Full Moon. Measurements
of about 200 galaxies must be averaged together to detect the
gravitational distortion signal, so the smallest area for which the
mass can be imaged is about 0.2% that of the Full Moon. The resulting
pictures are unacceptably blurred and are too grainy for many
purposes. For example, only the very largest lumps of matter (the
biggest clusters of galaxies) can be spotted in such maps with any
confidence. A second problem is that many of the distant galaxies
whose distortion is measured lie in front of many of the mass lumps
which one would like to map, and so are unaffected by their
gravity. To make a sharp image of all the mass in a given
direction requires more distant sources and requires many more of
them. MPA scientists Ben Metcalf and Simon White have shown that radio
emission coming to us from the epoch before the galaxies had formed
can provide such sources.

About 400,000 years after the Big Bang, the Universe had cooled off
sufficiently that almost all its ordinary matter turned into a
diffuse, near-uniform and neutral gas of hydrogen and helium. A few
hundred million years later gravity had amplified the non-uniformities
to the point where the first stars and galaxies could form. Their
ultraviolet light then heated the diffuse gas back up again. During
this reheating and for an extended period before it, the diffuse
hydrogen was hotter or cooler than the radiation left over from the
Big Bang. As a result it must have absorbed or emitted radio waves
with a wavelength of 21 cm. The expansion of the Universe causes this
radiation to be visible today at wavelengths of 2 to 20 metres, and a
number of low-frequency radio telescopes are currently being built to
search for it. One of the most advanced is the Low Frequency Array
(LOFAR) in the Netherlands, a project in which the Max Planck
Institute for Astrophysics is planning to take a significant role,
together with a number of other German institutions

The pregalactic hydrogen has structures of all sizes which are the
precursors of galaxies, and there are up to 1000 of these structures
at different distances along every line of sight. A radio telescope
can separate these because structures at different distances give
signals at different observed wavelengths. Metcalf and White show that
gravitational distortion of these structures would allow a radio
telescope to produce high-fidelity images of the cosmic mass
distribution which are more than ten times sharper than the best that
can be made using galaxy distortions. An object similar in mass to our
own Milky Way could be detected all the way back to the time when the
Universe was only 5% its present age. Such high-resolution imaging
requires a BIG telescope array, densely covering a region about 100 km
across. This is 100 times the size planned for densely covered central
part of LOFAR, and about 20 times bigger than densely covered core of
the Square Kilometre Array (SKA) the biggest such facility currently
under discussion. Such a giant telescope could map the entire
gravitating mass distribution of the Universe, providing the ultimate
comparison map for images produced by other telescopes which highlight
only the tiny fraction of the mass which emits radiation they can
detect.

We don't have to wait for the giant telescope to get world-beating
results from this technique, however. One of the most pressing issues
in current physics is to gain a better understanding of the mysterious
Dark Energy which currently drives the accelerated expansion of our
Universe. Metcalf and White show that mass maps of a large fraction of
the sky made with an instrument like SKA could measure the properties
of Dark Energy more precisely than any previously suggested method,
more than 10 times as accurately as mass maps of similar size based
on gravitational distortions of the optical images of galaxies.